Maxwell’s Silver Hammer of the Gods

In 1830, Michael Faraday
submitted his most famous
piece of scientific legislation‚
and this bit of genius described
the physics that allows guitar
pickups to exist. After ratification,
it became formally known
as the Faraday-Maxwell equation,
and it’s part of the four
coolest of Maxwell’s Equations.
Here it is (don’t worry, there
won’t be a test):

The basic idea is that if you
have a loop of wire, and you
can get a magnetic field inside
the loop to change, the changing
magnetic field will create a
changing voltage across the ends
of the wire. In a guitar pickup,
a permanent magnet creates a
static field that is modulated
(changed) by the movement of
a steel string. The steel part is
important: The string has to be
magnetic to disturb the field
created by the magnet. That’s
why you don’t see traditional
pickups for nylon-string guitars.
You may have also noticed
that pickups (or at least the
single-coil variety) provide equal
opportunity to stray magnetic
fields from transformers, fluorescent
lights, and such.

In the case of a typical guitar
pickup, a single loop of wire
will only provide a very tiny
voltage but, as shown below, we
can keep wrapping more turns
around the magnet to increase
the signal level. (See Fig. 1)

A typical pickup has thousands
of turns of very thin, insulated
wire. Increasing the number
of turns allows us to get a larger
signal for our amplifier to work
with, but as with everything,
there can be too much of a good
thing. As the turns pile on layer
after layer, it becomes like placing
a capacitor in parallel with
the pickup. You will hear people
talk about vintage handwound
pickups and how they differ
from the modern machine-wound
counterparts. One difference
is that the machine is able
to lay down more even layers,
while handwound pickups tend
to be more “scramble wound.”
The two different methods will
have a different capacitance.
Winding capacitance also
explains why low-output pickups
are typically brighter than the
high-output variety.

In addition to the winding
capacitance, the loops of
wire create an inductor, and
the resistance of the wire creates
(you guessed it) a resistor.
So, our little pickup is a
resonant RLC circuit “all by
itself.” Resonant circuits have a
center frequency and Q associated
with them. In general,
low-output pickups will have
a high Q at a high frequency,
and as you move to high-output
pickups, the Q and center frequency
both are reduced. In
the middle of the output range
is where you get different midrange
“honk.” The honk occurs
when the Q is still high, but
the frequency is reduced to the
midrange band.

So what does all this mean to
our quest for tone? Every signal
source has something called
output impedance. Sources with
a low output impedance don’t
really care what you hang on
the output. But a pickup has
a high output impedance and
thus is sensitive to loading by
pots, cables, and input stages.
This means if we want to hear
exactly what our pickup sounds
like, we would need to take the
volume and tone controls out
of our guitar and place a high-input
impedance buffer (10
Meg or more) a few inches away
from the pickup. Chances are
you wouldn’t like the unmolested
sound of the pickup because
we’re accustomed to hearing the
effect of all the stuff hanging
on the output. You probably
want a little bit of “tone suck,”
whether you know it or not.

One thing to keep in mind
is that once our signal path hits
a buffer, all of the cables and
input stages from that point on
don’t interact with our pickup.
This is the source of a lot of
confusion and attribution of
magic to certain combinations
of guitars, cables, pedals, and
amps. The permutations and
resulting confusion escalates
when you throw in true bypass
pedals. Assuming that your
signal is unbuffered, true bypass
removes the pedal from the
signal path but exposes your
pickup to more cables and a
different input stage. See Fig. 2.

Everything we connect
to our guitar alters the
tone—sometimes for better
and sometimes for worse. The
better and worse may flip-flop
for different guitars or even different
pickup combinations on
the same guitar. This is why you
will get more consistent results
by connecting your guitar to a
buffer as the first pedal or by
putting all of your effects in
the effect loop of an amp. But
consistent isn’t necessarily better—
you may miss that magic
combination. Don’t be afraid to
experiment, as there are only a
few things you can do that will
actually damage your gear. And
there are no wrong answers.

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